REPTILES AND BIRDS 135

there were any such intermediate forms they ought to
be more abundant as fossils than the typical reptiles or
typical birds :—

" An animal in the process of acquiring the power of flight
is peculiarly liable to meet with fatal accidents. Human ex-
perience in aviation demonstrates this. The acquisition of wings
by the accumulation of variations or mutations must in each
case have taken many thousands of years. For a considerable
part of this period the casualties as the result of accidents among
the animals so evolving must have been exceedingly numerous.
In consequence the deposits laid down during the period in ques-
tion should contain many fossils of these incipient flying
animals : the Devonian should hold thousands of fossils of what
may be termed pro-insects, the Trias a multitude of those of
pro-pterosaurs, the Trias and Lower Jura a great many of those
of pro-Aves, and the Eocene a large number of those of pro-
Chiroptera. It is submitted that these pro-creatures exist only
in the imagination of evolutionists " (D., p. 136).

In all this there is a serious fallacy—the notion that
a species imperfectly adapted to its surroundings is
more likely to be preserved as a fossil than one well-
adapted. I have tried to express the real state of things
in a series of population-graphs of a very diagrammatic
kind (Fig. 26), in which the numbers of a species are
indicated by vertical measurements and the passage of
time by horizontal measurements (left to right). The
graphs are smoothed, temporary fluctuations being
ignored. The straight and horizontal line AB denotes
a stable species, in which death-rate and birth-rate just
balance and the average numbers remain unchanged.
AC is the graph of a species so completely out of har-
mony with its surroundings that death-rate greatly
exceeds birth-rate, and it nose-dives to swift extinction.
AD starts in similar plight, but the destructive agents
are selective and the death-rate begins to fall off (giving
a curve with upward concavity) though not quickly
enough to avoid extinction. In AEF we see a case in
which selection results in adaptation and the nose-dive